Conventionally, an electronic device, such as a server or a personal computer (PC), may be equipped with a blower for blowing air into the inside of the device thereby releasing heat to the outside in order to prevent an increase in temperature of the inside of the device due to heat generation by a processor and the like.
A blower produces noise (wind noise) due to a vortex of air occurring near blades. The noise from the blower increases in proportion to a volume of air flow; therefore, when a higher volume of air flow is obtained by increasing a rotation frequency of the blower, this increases noise as well. Specifically, it is known that noise from a blower is proportional to the fifth to sixth power of an axial rotation frequency of the blower.
In recent years, electronic devices are set up in not only particular places, such as a computer room, but also general offices; therefore, there is a growing awareness of noise reduction. Thus, how to reduce noise from a blower is one of the important issues.
As a way to reduce blower noise, for example, there is known a method to monitor the temperature of a heat generating body and the environmental temperature and change a rotation frequency of a blower depending on these temperatures, thereby controlling not to increase the blower noise more than necessary. Incidentally, the control of blower rotation frequency is made by modulating the voltage or the pulse width (a PWM value) in pulse width modulation (PWM), thereby controlling energy to be supplied to a motor of the blower.
Meanwhile, in recent years, with the trend toward miniaturized, thinner electronic devices, some electronic devices have no ventilation flue within the device; therefore, there have been an increasing number of electronic devices that can have only a small blower. Furthermore, with the trend toward high-speed, high-performance electronic devices, an amount of heat generation of an electronic device tends to be increasing every year. Accordingly, electronic devices are designed to use a plurality of piled-up blowers so as to cool the inside of the electronic device sufficiently even if the electronic device can have only small blowers. For example, there is a technology to install two blowers, which are the same in air-flow direction but different in rotation direction, to be arranged in series in the air-flow direction so as to cool the inside of an electronic device sufficiently.
In this technology, to suppress noise produced by the blowers, rotation control of each of the two blowers is individually performed depending on a change in environmental temperature.
Incidentally, there is also known a technology to control a phase difference between two front and back blowers so as to reduce noise from the blowers on the basis of the number of blades, a rotation frequency, and a distance between rotor blades.    Patent document 1 Japanese Laid-open Patent Publication No. 02-238195    Patent document 2 Japanese Laid-open Patent Publication No. 2000-136798    Patent document 3 Japanese Laid-open Patent Publication No. 2008-25983
However, in the above-mentioned conventional technologies, there still remains a problem that noise produced by a blower cannot be suppressed. For example, a noise level of noise produced by a blower varies according to a system impedance of an electronic device, a PQ characteristic of the blower with respect to each ratio of respective rotation frequencies of two fans of the blower, and a load noise characteristic with respect to each rotation frequency ratio. However, in the above-mentioned technologies except Japanese Laid-open Patent Publication No. 2010-272704, the rotation control for suppressing noise produced by a blower is performed without consideration of this. Furthermore, in Japanese Laid-open Patent Publication No. 2010-272704, it is effective in blowers having the same shape and the same characteristic in rotation direction; however, there may be no effect when fans, which are the same in air-flow direction but different in rotation direction, are arranged in series in the air-flow direction. Accordingly, in the above-mentioned conventional technologies, suppression of noise is still insufficient, and further noise suppression is expected. Incidentally, the PQ characteristic indicates a relationship between static pressure and air flow. Furthermore, the load noise characteristic indicates a relationship between air flow and noise level. Moreover, the system impedance is also referred to as a “pipeline resistance”.
Incidentally, in the above-described technology to control a phase difference between front and back blowers, frequency analysis of noise is performed when a phase difference is controlled. Therefore, if there are multiple noise sources other than the blowers, such as a processor and a hard disk drive (HDD), in an electronic device, it is difficult to extract only noise produced by the blowers, objects to be controlled, and perform frequency analysis of the extracted noise.
The present invention has been made in view of the above, and an object of the present invention is to provide a blower control device, blower control method, and blower control program capable of suppressing noise.